The present invention describes a transparent plate made of lithium aluminosilicate glass ceramic showing high transmission, a process for making same and transparent plate laminates comprising at least one plate of the lithium aluminosilicate glass ceramic according to the invention and the use thereof as armored glass or bullet-proof vest.
Until now, the inherent color of transparent glass ceramics has been too strong. The reasons for the inherent color of transparent glass ceramics can vary. The constituents of the raw material mixtures for the melts contain the coloring element Fe as an impurity. The use of the refining agents Sb2O3 and CeO2 also results in a slight inherent color. The described brownish-yellow intrinsic color of the transparent glass ceramics is based substantially on electronic transitions occurring on colored complexes which absorb in the region of visible light and in which the component required for nucleation, namely the Ti ion, takes part. The most frequent absorbing color complex stems from the formation of adjacent Fe and Ti ions between which electronic charge transfers take place. Sn—Ti complexes also impart an inherent color. The Fe/Ti color complexes lead to a red-brown discoloration and the Sn/Ti color complexes to a yellow-brown one. The formation of these adjacent color complexes takes place already during the cooling of the parent glass and particularly during the subsequent ceramization of the glass ceramic. In the melt, the ions are still uniformly distributed, but during cooling at high temperatures and during ceramization they preferably bind to each other. As a result, during the ceramization of the transparent glass ceramics, the inherent color intensifies very markedly compared to that of the parent glass. By absorption in the short-wave region of the visible spectrum, transparent flat glasses and particularly the glass ceramics produced therefrom assume a pronounced inherent color which increases considerably with thickness.
It is known that the inherent color of glass ceramics can be reduced by overcoloring. The principle of overcoloring an undesirable color tinge naturally leads to stronger light absorption thus reducing the overall transmission, because the absorptions taking place are neutralized by the absorptions of complementary light portions by the overcoloring agent.
Glass ceramic plates find use in, among other applications, bullet-proof glass plates. In the production of such glass plates, several different glass or glass ceramic layers and plastic sheets are linked. The temperature- and pressure-controlled process of linking the individual layers and plastic materials to each other, in particular, is time-consuming and cost-intensive. The many interfacial transitions between glass plates and plastic materials result in poor transmission characteristics which may lead to the formation of interference fringe patterns in the form of Newton fringes. Also, the large amount of glass, namely the high number of glass plates in the known bullet-proof plates results in their exhibiting a very high weight per unit area. The high weight per unit area leads to a pronounced constructive cost of installation and vitrification.